The invention relates to a lighting system comprising a body which, in operation, emits visible light and comprising an envelope which transmits light.
Lighting systems of the type mentioned in the opening paragraph are known per se and comprise, for example, incandescent lamps, such as decorative lamps, for example so-called carbon filament lamps, wherein the incandescent body comprises a spirally wound wire of carbon on a carrier material. Other examples of such lighting systems include lamps provided with a light-emitting body in the form of cross-shaped, star-shaped elements or other decorative or devotional elements (for example the letters xe2x80x9clovexe2x80x9d).
It is a drawback of such lighting systems that the known lighting system has a relatively low luminous efficacy and a limited service life.
It is an object of the invention to provide a lighting system of the type mentioned in the opening paragraph, wherein the luminous efficacy and the service life are improved. To achieve this, the lighting system in accordance with the invention is characterized in that
the lighting system comprises at least one opto-electronic element which, in operation, emits light in a first wavelength range,
in that the body is provided with conversion means for converting light of the first wavelength range to light of a second wavelength range, and
in that the envelope is provided with a coating which at least partly reflects light of the first wavelength range.
Opto-electronic elements include electro-luminescent elements, such as light-emitting diodes (LEDs). Such opto-electronic elements are used as a source of white or colored light for general lighting purposes and as a source of colored or white light in signal lamps, for example in traffic control systems, vehicles, aircraft or other means or systems of transport. In recent years, apart from yellow and red light emitting diodes on the basis of GaP, efficient blue and green light emitting diodes on the basis of GaN have been developed. Such opto-electronic elements have a relatively high luminous efficacy (xe2x89xa720 lm/W) and a relatively long service life (xe2x89xa775,000 hours). By way of comparison, a 75 W carbon filament lamp has a luminous efficacy of approximately 2 lm/W and an average service life of less than 1,000 hours.
In operation, in the known lamp light is generated in that the (incandescent) body in a (vacuumtight) envelope is heated by means of an electric current, causing said body to emit light at a high temperature. In the known lamp, the (incandescent) body constitutes the so-called primary light source. In accordance with the invention, the (incandescent) body of the known lighting system has been replaced by a combination of at least an opto-electronic element and (a body provided with) conversion means which convert light, which is emitted by the opto-electronic element in a first wavelength range, to light in a second wavelength range. In the lighting system in accordance with the invention, the opto-electronic element is considered to be a primary light source, and the conversion means are considered to be a secondary light source. The conversion means are excited by light originating from the opto-electronic element. A part of this light is converted by the conversion means, for example via a process of absorption and emission, to (visible) light in the second wavelength range.
In accordance with the invention, the light-transmitting envelope further comprises a coating which at least partly reflects light of the first wavelength range. As a result, it is achieved that light of the first wavelength range, which originates from the opto-electronic element and which is not directly absorbed by the conversion means and converted to light in the second wavelength range, is reflected by the reflective coating applied to (an inner surface of) the envelope and still absorbed by the conversion means and converted to light in the second wavelength range. Light originating from the conversion means is allowed to pass by the envelope coated with the reflective layer. The reflective coating causes the body which, in principle, is irradiated only from below to be a homogeneously radiating body.
In accordance with the measure of the invention, a high-efficacy lighting system having a relatively long service life is obtained. In the lighting system in accordance with the invention, the envelope no longer serves as a vacuum envelope but as a means for providing the reflective coating. As a result of the fact that the lamp in accordance with the invention no longer has a vacuum envelope, said lamp in accordance with the invention is also safer to use. The envelope provided with the reflective coating may also be omitted, if so desired. The lighting system then comprises a combination of at least an opto-electronic element and (a body provided with) conversion means.
Preferably, the conversion means comprise a luminescent material. Such materials are particularly suitable because they generally have a high quantum efficiency and a high lumen equivalent (expressed in lm/W), so that a high luminous efficacy of the lighting system is achieved. In addition, a great variety of (stable) inorganic and organic luminescent materials (phosphors) is known, which makes it easier to choose a suitable material for achieving the object of the invention (improving the color rendition).
The luminescent material can preferably be excited by light originating from the wavelength range of 400 to 500 nm. As a result of this sensitivity, the luminescent material can particularly suitably be used to absorb, in particular, blue light. This absorbed light is very efficiently converted by the luminescent material to visible light in the further wavelength range, for example green or red light. The desired color temperature of the lighting system depends on the application. For a lighting system in the form of a look-alike of a carbon filament lamp, a relatively low color temperature is desired. For other applications, light having a high color temperature may be obtained.
A particularly attractive embodiment of the lighting system in accordance with the invention is characterized in that the at least one opto-electronic element is at least one solid-state electroluminescent element, which preferably comprises a blue light-emitting diode, and that the conversion means comprise a luminescent material for converting a part of the light emitted by the blue lightemitting diode to red light.
Preferably, the maximum of the spectral emission of the blue light-emitting diode lies in the wavelength range from 460 to 490 nm, and the maximum of the spectral emission of the red light-emitting luminescent material lies in the wavelength range from 610 to 630 nm.
In a favorable embodiment of the lighting system in accordance with the invention, a luminous flux of the opto-electronic element is, during operation, at least 5 lm. The invented lighting system enables a continuous, uniform illumination with a high intensity to be obtained. It has been found that opto-electronic elements having a luminous flux of 5 lm or more can only be applied in an efficacious manner if the lighting system comprises heat-dissipating means. Only lighting systems provided with opto-electronic elements having such a high luminous flux can replace customary incandescent lamps. A particular aspect of the invention resides in that the heat-dissipating means dissipate the heat generated during operation of the lighting system to a lamp cap of the lighting system and/or the mains supply connected thereto.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.